It is well known to plasma etch layers on a semiconductor substrate to fabricate electronic devices. Plasma etching is the removal of material by reaction with chemically active gases created by an RF power induced glow discharge environment in an etching chamber. As compared with wet chemical etching, it is dry, cleaner, more economical, free of volumes of waste to be disposed of and, when properly controlled, capable of sharper etching with less undercutting.
Plasma etching is controlled by a number of parameters including the nature of the etching gas, the RF power level used, the gas flow rate and chamber pressure, the temperature, and the load or volume of material to be etched. Because of the many variables, empirically setting process specifications and obtaining run-to-run reproducibility has been very difficult.
In some etching processes the selectivity and anisotropy are merely adequate; it is desirable to etch only until a pattern is defined. Over etching leads to problems such as undercutting and etching of the underlying material. Consequently, it is desirable to stop the etching process as close to the point of completion as possible.
Plasma etching chambers typically have viewing windows of optical quartz. Experienced operators are sometimes able to detect a color intensity change of the plasma during the etching process as the products generated often change when etching is complete, indicating that an end point has been reached and exceeded. However, this kind of control is highly subjective. Another very subjective control for an experienced operator is the visual detection of the color change in a non-absorbing film as it is etched away or, similarly, noting the disappearance of a metallic film.
One automated method for end point detection in a plasma etching process is described in U.S. Pat. No. 4,198,261 to Busta et al. That patent describes the use of an optical technique in which light is beamed on a layer to be etched and the resulting reflected and refracted beam is detected. Different values of light intensity can be detected when the desired layer is etched and the underlying layer receives the light beam. The change in light intensity may be used to trigger the end of the etching process. Unfortunately, background signals tend to mask the reflected light signal resulting in difficult or inaccurate detection of the end point.
Other end point detection methods such as spectroscopic and laser interferometric techniques are also well known. However, spectroscopy requires accurate monitoring of an atomic emission line which is difficult to accomplish in a plasma due to the variations in the emission intensity with fluctuations or drifts in the plasma parameters as well as the chemical constituents. This usually results in an end point that is not sharply defined. The laser interferometric techniques are not applicable to many critical etching steps (such as aluminum) since they rely on the interference of two lightwaves, one reflected from the top and the other from the bottom of the film to be etched. Results require even greater interpretation compared to spectroscopic techniques.
Accordingly, there is a need for an effective, accurate method of detecting the end point of a plasma etching operation.